It is well known that free space electric fields are emitted from many different types of sources. For example, it is known that organs in the human body, such as the heart and the brain, produce electric fields. Also, electric fields are produced whenever vehicles and machinery are operated. Further, they are emitted from many other diverse types of sources, such as active corrosion sites. In each case, for a variety of different reasons, it may be desirable to detect and measure these electric fields.
At low frequency, antennas that are effective for receiving signals from free space electric fields commonly have the properties of a pure voltage source that is in series with a very small capacitance. Consequently, electric field antennas will typically have an ultrahigh impedance that will often be in a range between ten and one hundred giga ohms (10–100 GΩ). Due to this ultrahigh impedance, any amplifier that may be used to amplify signals of the electric field should have an ultrahigh input impedance.
By definition, “matching” involves adjusting a load impedance (e.g. amplifier) to the source impedance (e.g. antenna) so that maximum power is received. Stated differently, matching is necessary so that there is minimal reflection loss due to a mismatch between the source and the load. Accordingly, in order for an amplifier (load) to achieve high coupling efficiency and thereby effectively amplify the signals of an electric field (source) it is often necessary for the amplifier to have an ultrahigh input impedance. Additionally, this input impedance must be purely capacitive. The problem here, however, is that any resistive path of even very high resistance value at the amplifier input, will also act as a short to ground. This will then effectively insert an RC filter into the amplifier input circuit that will adversely affect the coupling between the antenna and the amplifier. For example, for an amplifier having an input impedance of 100 GΩ, and a resistive path to ground of 1 GΩ at the amplifier input, only 1% of the signal will be coupled into the amplifier. Thus, it is generally desirable to remove resistive paths to ground at the amplifier input.
Although the removal of resistive paths from an amplifier input may improve the coupling efficiency of the amplifier, it will not necessarily solve all of the problems that are presented by an ultrahigh input impedance amplifier. Specifically, the removal of resistive paths from the amplifier input still does not account for the amplifier's inevitable input bias current. In general, the input bias current is a nonsignal current that is either d.c. in nature or has a very low frequency. In any event, unless the input bias current is somehow accounted for, it will flow onto the amplifier input capacitance and onto the source capacitance, and thereby produce a continual drift in the input signal level. This can then lead to saturation of the amplifier. For example, it can be shown that for an input bias current of 50 fA and an input capacitance of 10 pF, an amplifier with a gain of 1000 would saturate in approximately 2 seconds.
In light of the above it is an object of the present invention to provide a system and method for stabilizing an amplifier that enables an ultrahigh input impedance amplifier to be connected directly to a free space electric field antenna without the provision of a current path to ground to shunt the input bias current away from the amplifier input. Another object of the present invention is to provide a system and method for stabilizing an amplifier that allows the amplifier to be connected to ultrahigh impedance sources with high coupling efficiency. Still another object of the present invention is to provide a system and method for stabilizing an amplifier that is effectively easy to use, relatively simple to manufacture, and comparatively cost effective.